Field of the Invention
This invention relates to a spark plug in which an erosion-resistant tip is secured to a front end of a center electrode by means of welding.
In a center electrode for a spark plug for use in an internal combustion engine, in order to provide the center electrode with heat-and oxidation-resistant property, the center electrode has a nickel-based metal in which a copper core is embedded as a heat-conductor core.
Further, a tip which is made of precious metal such as platinum-based alloy is welded to an front end of the center electrode so as to improve spark-erosion resistance.
In related prior arts, U.S. Pat. No. 3,146,370 suggests a center electrode for a spark plug in which a tip is welded to a firing portion of the center electrode in which the tip has a cobalt (Co) core cladded by an iridium (It) sheath.
In a Japanese Patent Application No. 1-314315 filed by the applicant of the invention, an inventor suggests an optimal dimensional relationship between a tip and a recess in which an iridium-based tip is fit in a recess provided at a front end surface of a center electrode, and the tip is secured to an outer wall of the recess by means of laser or electron beam welding.
With high speed and high power requirement of the internal combustion engine, the front end of the center electrode tends to be exposed to higher ambient temperature. In order to protect the tip against thermal deterioration, it is necessary to prevent the temperature of the tip from abnormally rising. The iridium-made tip, a melting point of which is as high as 2500.degree. C., has remarkable spark-erosion resistant property. The tip, however, deteriorates due to evaporation when oxidized by being exposed to the high ambient temperature of more than 900.degree. C.
In addition, a distance between a rear end of the tip and a front end of the copper core is 1.0 mm or more, and therebetween lies a part of the nickel-alloyed sheath which is relatively poor in thermal conductivity.
This blocks to thermally transmit a sufficient amount of heat from the tip to a rear end of the center electrode by way of the copper core so as to deteriorate a heat-conductive property when the tip is exposed to a combustion chamber of the internal combustion engine. For this reason, temperature of the tip is likely to excessively rise particularly when the engine runs at high speed with high load.
When the tip is bonded directly to the front end of the copper core by means of electrical resistance welding, the front end of the copper core is likely to outcrop from the nickel-alloyed sheath due to their thermal expansional difference, and oxidized in the higher atmospheric ambience.
Therefore, it is an object of the invention to provide a center electrode for a spark plug which is capable of effectively preventing the temperature of a tip from abnormally rising so as to keep the tip firmly in place without falling the tip off the recess by thermal damage of the welding portion, and contributing to an extended service life with relatively low cost. In a center electrode for a spark plug, a tip is fitted in a recess provided on a front end surface of the nickel-alloy metal, and the tip is in such a manner that a front end of the tip is protracted from the recess, and an outer surface of the tip is bonded to an inner surface of the recess by means of laser or electron beam welding.